1. Field of the Invention
The present invention relates to an optical transmission system, more particularly, to an optical Wavelength-Division-Multiplexing (i.e., “WDM”) system. Furthermore, particularly, the invention relates to a WDM device and a WDM system and transmission network using the same.
2. Description of the Related Art
The existing transmission systems confront insufficient transmission capacities as the amount of information used by individuals rapidly increases in the Internet, multimedia communication, electronic commerce, and so on. In order to cope with the above situation, a WDM mode tends to gradually spread. The WDM mode is one of transmission modes for transmitting optical signals, by which a plurality of optical signals having wavelengths different from one another are simultaneously guided to a piece of optical fiber. In the WDM mode, wavelength-division-multiplexing means multiplexing the optical signals with different wavelengths into the one piece of optical fiber. On the contrary, wavelength-division-demultiplexing means dividing the optical signals multiplexed in the one piece of optical fiber.
This WDM transmission technology is a method which can increase transmission capacity in the easiest way in the optical communication, and a WDM system adopting the WDM transmission technology is getting rapidly commercialized. This movement is spreading to the overall transmission network so that the range thereof is extending from a metro area to an access area as well as the backbone network.
A basic construction of a typical WDM system in use for the backbone network is illustrated by a 4 channel WDM system for the sake of convenience. Channel transmitters generate optical signals of wavelengths according to their channels based upon the WDM standards in response to electric signals subjected to transmission according to their channels, and apply the optical signals to a WDM multiplexer. The WDM multiplexer wavelength-division-multiplexes the optical signals of channels which are transformed by the channel transmitters into one optical fiber, and transmits the WDM signal into an optical transmission channel. The optical transmission channel is provided with optical amplifiers for compensating losses occurring to the WDM optical signal while it propagates along the optical fiber and optical devices. A WDM demultiplexer receives the WDM optical signal propagated as above and divides the WDM optical signal into optical signals according to the wavelengths via wavelength-division-demultiplexing. Channel receivers transform the optical signals divided with the WDM demultiplexer into electric signals via photoelectric transformation.
In the WDM system in use for the backbone network as above, those portions common to each wavelength such as the WDM multiplexer, the optical amplifier and the WDM demultiplexer are necessarily installed at the initial stage.
However, compared to the WDM system used in the backbone system, the WDM system used in the metro area or the access area requires a comparatively low cost, has a small amount of system capacity and is relatively restricted in use of an Erbium-Doped Fiber Amplifier (i.e., “EDFA”).
Therefore, in an area such as the metro/access area requiring a relatively cheap system compared to the backbone network, it is burdensome in respect of the initial installation cost to install the expensive WDM system such as the WDM system used in the backbone network and thus it is difficult to adopt the WDM mode.